1. Field of the Invention
The present invention relates to an imaging system.
2. Description of the Related Art
Along with the recent increase in the number of pixels of an image sensing apparatus in an image sensing system, demand for quick processing of acquired image signals is increasing. To meet this demand, there is proposed a technique of providing A/D converting units, which have conventionally been provided outside the image sensing apparatus, to the respective columns of the pixel array inside the image sensing apparatus. Such an image sensing apparatus is called a column AD type image sensing apparatus. The column AD type image sensing apparatus will be described in detail below with reference to the accompanying drawings.
As shown in FIG. 8, a conventional imaging system S1 comprises an image sensing apparatus 700, A/D converting unit 4, timing signal generating circuit 5, and signal processing unit 7.
The image sensing apparatus 700 photoelectrically converts an object image formed on its image sensing surface to generate an analog image signal, and supplies it to the A/D converting unit 4. The timing signal generating circuit 5 supplies timing signals to the A/D converting unit 4 and signal processing unit 7. The A/D converting unit 4 A/D-converts the analog signal to generate a digital image signal in synchronism with the timing signal, and supplies it to the signal processing unit 7. The signal processing unit 7 processes the digital image signal in synchronism with the timing signal, and outputs the processed digital image signal to the succeeding stage.
As shown in FIGS. 9 and 10, there is also proposed an imaging system S2 having a column AD type image sensing apparatus 800 in which A/D converting units 811 to 814 are provided to the respective columns of a pixel array PA. The column AD type image sensing apparatus 800 can quickly read out pixel signals and easily perform arithmetic processing for the readout signals.
The A/D converting units 811 to 814 receive signals from the pixels of the respective columns of the pixel array PA.
The image sensing apparatus 800 shown in FIGS. 9 and 10 drives all of the plurality of A/D converting units 811 to 814. This increases power consumption for driving the plurality of A/D converting units 811 to 814, which may lead to heat generation by the plurality of A/D converting units 811 to 814. In this case, the characteristics of the image sensing apparatus 800 may degrade.
More specifically, the dark current of a photodiode, which is contained in each pixel of the pixel array PA in the image sensing apparatus 800, is generally said to double upon a temperature rise of 8° C. In addition, defects in a pixel signal attributed to the dark current of a photodiode and fixed pattern noise due to a variation in the dark current of the photodiode are widely known to exhibit a temperature characteristic that they increase along with a temperature rise.
That is, when the plurality of A/D converting units 811 to 814 generate heat, the number of defects in a pixel signal attributed to the dark current of each photodiode and fixed pattern noise due to a variation in the dark current of the photodiode increases. This may degrade the quality characteristics of an image corresponding to the image signal acquired by the image sensing apparatus 800.